Process for removal of cationic impurities from technical grade phosphoric acid

ABSTRACT

Technical grade phosphoric acid is purified of cationic impurities by adding thereto alkali or ammonium ions and a water soluble organic solvent whereby a flaky, easily filterable precipitate is formed which is subsequently separated from the filtrate. The filtrate is passed through a highly acidic cationic exchange resin in the H+ form. The organic solvent is distilled from the filtrate to produce purified phosphoric acid. The source of alkali ions include gaseous ammonia, alkali and ammonium hydroxide and/or alkali and ammonium salts with the addition being of from 0.06 to 0.12 mols of alkali ions per mol of P2O5. The water soluble organic solvent is preferably isopropanol and is added in an amount of from two to five times the amount with respect to the raw phosphoric acid.

United States Patent Frankenfeld et a1.

3,764,657 Oct. 9, 1973 [73] Chemische Fabrik, Budenheim,

Germany [22] Filed: Sept. 21, 1971 [21] Appl. No.: 182,534

[30] Foreign Application Priority Data Oct. 14, 1970 Germany P 20 50406.7

52 US. Cl. 423/321 [51] Int. Cl COlb 25/16 [58] Field of Search23/165,165 B, 165 C;'

[56] References Cited UNITED STATES PATENTS 3,650,691 3/1972 Reulecke eta1. 23/165 1,929,441 10/1933 Milligan 23/165 PROCESS FOR REMOVAL OFCATIONIC IMPURITIES FROM TECHNICAL GRADE PHOSPHORIC ACID Inventors:Klaus Frankenfeld, Kirberg/Taunus; Karl Giitzmann, Budenheim/Rhine, bothof Germany Assignee:

FOREIGN PATENTS OR APPLICATIONS 464,370 4/1937 Great Britain 23/165Primary Examiner-Edward J. Meros Assistant ExaminerGregory A. HellerA!t0rney-Marn & Jangarathis [57] ABSTRACT Technical grade phosphoricacid is purified of cationic impurities by adding thereto alkali orammonium ions and a water soluble organic solvent whereby a flaky,easily filterable precipitate is formed which is subsequently separatedfrom the filtrate. The filtrate is passed through a highly acidiccationic exchange resin in the 1-1-1- form. The organic solvent isdistilled from the filtrate to produce purified phosphoric acid. Thesource of alkali ions include gaseous ammonia, alkali and ammoniumhydroxide and/or alkali and ammonium salts with the addition being offrom 0.06 to 0.12 mols of-alkali ions per mol of P 0 The water solubleorganic solvent is preferably isopropanol and is added in an amount offrom two to five times the amount with respect to the raw phosphoricacid.

6 Claims, No Drawings PROCESS FOR REMOVAL OF CA'IIONIC IMPURITIES FROMTECHNICAL GRADE PHOSPHORIC ACID This invention relates to thepurification of phosphoric acid, and more particularly to a process forremoving cationic impurities from phosphoric acid produced by the wetprocess.

BACKGROUND OF THE INVENTION Phosphoric acid is generally produced inaccordance with two processes. In one process, phosphoric acid isproduced from phosphate ore by contact with a strong mineral acid, suchas sulfuric acid. Calcium in the phosphate ore is generally separated inthe process as gypsum whereas the other constituents in the phosphateore remain in the phosphoric acid asdissolved solids or soluble salts tothe extent they cannot form slightly soluble compounds in the stronglyacid medium with either sulfuric or phosphoric acid. In many instances,these impurities interfere with the further use of the phosphoric acid,and often the employment of technical grade acid for many purposes isrendered impossible by these impurities. Efforts have not been lacking,therefore, to free the phosphoric acid or its alkaline salts of suchimpurities. In one long-used purification method the phosphoric acid isneutralized'with alkalis.

Accordingly, the impurities dissolved in the raw acid are precipitatedas slightly soluble salts or metalhydroxides. This method is thereforesuitable as a'purifying method only if the phosphoric acid is to beconverted into its alkali salts.

lf compounds other than alkali phosphates are to be made from thephosphoric acid, then purification methods are required whereby the freeacid remains. As disclosed in German Pat. No. 648,295, there isdescribed a method of purifying raw phosphoric acid by treating thephosphoric acid with an organic solventin the presence of an alkalicompound whereby-.theimpurities such as calcium, iron, aluminum,chromium, vanadium and fluorine are removed as a precipitate. Mentionedare methyl and ethyl alcohol and acetone; Alkaline compounds used in theprocess include free alkali and ammonium base, the salts of sulfuricacid,.

oxalic acid, carbonic acid, nitric acid and the like. The amount ofalkaline compound to be added is set optionally at percent with respectto the raw phosphoric acid. Because of the large amounts of alkalinecompounds required for purification, a partial desaturation of thephosphoric acid is effected. Thus, computation of the numerical valuesset forth in the examples, shows that about 18.5 percent of theavailable phosphoric acid is monobasically desaturated. Corresponding tosuch high level of partial desaturation, the yield of purifiedphosphoric acid is poor. Also, thephosphoric acid is contaminated withalkali compounds partially dissolved in the phosphoric acid.Additionally, the'impurities settle-out as viscous, green clumps of oilyappearance which only gradually become discrete. Such a slimyprecipitate can be separated only with great difficulty (e.g.decantation), so that a division of the phases is technologicallyuncontrollable because the solid phase constantly changes inconsistency.

OBJECTS OF THE INVENTION An object of this invention is to provide animproved process for purifying phosphoric acid.

Another object of this invention is to provide an improved process foreffectively removing cationic impurities from phosphoric acid producedby the wet acid process.

DETAILED DESCRIPTION OF THE INVENTION These and other objects of theinvention are accomplished by effecting the precipitation of cationicimpurities by using a water soluble organic solvent and alkali orammonium ions, where the ratio of ions amounts to 0.06 to 0.12 mole permol of P 0 The precipitate is filtered and the filtrate passed through ahighly acidic cationic exchange resin in the H form. The solvent isfractionated from the filtrate to reconstitute the phosphoric acid.Accordingly, the impurities are separated out as flaky, easily settlingand readily filterable solids.

The alkaline compounds to be added include alkali or ammoniumhydroxides, alkali or ammonium salts and gaseous ammonia. The amount ofalkali ions to be added is about 0.06 to 0.12 mols per mol of P 0 in theraw phosphoric acid. Isopropyl alcohol was found particularly useful asthe solvent. The amount of solvent added varies between about two tofive times the amount with respect to the raw phosphoric acid. A part ofthe added alkali or ammonium ions remains behind in the solventphosphoric acid phase as dissolved salts. As the case may be, dependingon the concentration of the phosphoric acid and the ratio to the aqueousacid to alcohol, the proportion of dissolved alkali or ammonium salts inthe purified phosphoric acid can reach 50 to 70 percent of the quantityadded.

The alkali or ammonium salts are removed according to the inventiveprocess by subsequently passing phosphoric acid through a highly acidiccationic exchange resin in-I-l form Surprisingly, it has been found thata phosphoric acid (diluted with an organic alcohol) having a H POcontent of up to 30 percent can be readily purified by such a resin withhigh utilization factor of the resin capacity. The surprising aspect ofthe invention in readily apparent when one considers that normally 10percent or less of the mineral acid is used for the purpose ofconverting charged, highly acidic cationic exchanger resin into the H+FORM. On the basis of this, one would have expected that from acids atsuch concentrations no cations would be exchanged for l-H- ions. Bymeans of the process of this invention not only are the alkalisremaining behind in the previously purified phosphoric acid removed andthe yield of free phosphoric acid thereby increased, but also othercations in the starting raw phosphoric acid and which were notdiminished or which were only partly reduced in amount by the precedingprecipitation procedure are completely removed.

The purified acid according to the process of the invention containsnone of the original l-, 2- and 3- valent cations present in the raw.acid in quantities demonstrable by the normal methods. Only the iron andchromium are sometimes perceptible in small traces. Naturally thepersisting remainder is dependent on the starting concentration. Theimpurities remaining behind lie in the order of magnitude of from 0.5 tol percent relative to the starting concentration. An acid purifiedaccording to the present invention for a P 0 content of 50 percent stillhas a iron content of from 0.001 to 0.003 percent and a chromium contentof 0.001 to 0.002 percent. Due to the small amounts of ammonium oralkali ions required to be added, the

I amount of the solid phase is smaller and can be sharply compressed andrinsed with alcohol.

Additionally, there is a considerable reduction in the P losses. Becauseof the small of ammonium ions introduced, there is a reduction in thesettling out ofthe solid phase since a flaky, easily filterableprecipitate is formed as distinguished from the viscous oily mass of theprior art. This flaky precipitate can be pressed out to large degree andrinsed with alcohol. P 0 losses are necessarily dependent on the levelof impurities in the raw acid, and vary between 3 to 6 percent withrespect to the quantity of P 0 in the starting acid.

In operation, a customary amount of raw acid is placed in a mixingvessel. The alkali ions are added in an amount, based on the P 0content, with continuous stirring. When using ammonium hydroxide orgaseous ammonia, a slight temperature elevation of from 5 to 8C. willoccur. After the alkali addition, an appropriate amount of isopropanolis added to the liquid with stirring. A fine, white to light grayprecipitate settles out which at once coalesces into rather largeflakes. This tendency is further reinforced by stirring for anadditional period of time of from 2 to 5 minutes. After transfer fromthe mixing vessel, the precipitate settles quickly to the bottom. Theliquid standing above is clear. The precipitate is separated from thesolution by filtration and is rinsed with about 10 percent of the amountof solvent medium. The filtrate is passed through a highly acidiccationic exchange resin in the H+ form and subsequently freed of solventin a distillation apparatus with the phosphoric acid remaining therein.The distilled solvent can be rinsed and recycled for additionalpurification operations. It will be understood that the process of theinvention can be conducted batchwise as well as continuously.

Numerous modifications and variations of the invention are possible inthe light of the above teachings and therefore the invention may bepracticed otherwise than as particularly described.

What is claimed is:

1. A process for purifying technical grade phosphoric acid whichcomprises:

a. adding alkali ions to said phosphoric acid, the amount of alkali ionsbeing from 0.06 to 0.12 mols per mol of P 0 in said phosphoric acid;

b. adding a water soluble organic solvent to said phosphoric acid, saidorganic solvent being added in an amount of from 2 to 5 times the amountof said phosphoric acid;

0. separating a precipitate resulting from steps a and d. passing thefiltrate of c through a highly acidic cationic exchange resin in the I-lform; and

e. distilling the solvent from the filtrate of step d.

2. The process as defined in claim 1 wherein the organic solvent addedis less than about 30 volume percent.

3. The process as defined in claim 1 wherein isopropanol is said watersoluble alcohol.

4. The process as defined in claim 1 wherein the alkali ion isintroduced in the form of gaseous ammonia.

5. The process as defined in claim 1 wherein the alkali ion isintroduced is ammonium hydroxide.

6. The process as defined in claim 1 wherein the alkali ion isintroduced in the form of an ammonium salt.

2. The process as defined in claim 1 wherein the organic solvent addedis less than about 30 volume percent.
 3. The process as defined in claim1 wherein isopropanol is said water soluble alcohol.
 4. The process asdefined in claim 1 wherein the alkali ion is introduced in the form ofgaseous ammonia.
 5. The process as defined in claim 1 wherein the alkaliion is introduced is ammonium hydroxide.
 6. The process as defined inclaim 1 wherein the alkali ion is introduced in the form of an ammoniumsalt.